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JAKE TAPPER, CNN ANCHOR: You know that? Do you?

[17:00:04](voice-over): To all the volunteers who have fought and will fight for our freedom, thank you.

(END VIDEOTAPE)

TAPPER: That's it for "THE LEAD." Here's Wolf Blitzer.

WOLF BLITZER, CNN ANCHOR: Happening now, breaking news. I'm with her. After delivering the news to Bernie Sanders, President Obama delivers his big endorsement of Hillary Clinton. The president says he's fired up to join the Clinton campaign out there on the campaign trail and the Democrats are lining up their other big guns for a hard- hitting campaign.

Distracted. Donald Trump is facing trouble within his own party. Key GOP leaders are still withholding their support, and House Speaker Paul Ryan says his antics are distracting. Can Trump patch things up with fellow Republicans?

An ISIS American. A Virginia man who joined ISIS and then defected appears in federal court on terror charges. Why the first American ISIS fighter is captured on the battlefield and could be a gold mine for U.S. intelligence.

I'm Wolf Blitzer. You're in THE SITUATION ROOM.

ANNOUNCER: This is CNN breaking news.

BLITZER: Our breaking news, President Obama endorses Hillary Clinton for president of the United States, saying he doesn't think there's ever been someone so qualified to hold that office. The president says he's fired up, and he'll start campaigning with Hillary Clinton next week in Wisconsin. Clinton says she's honored and that she, too, is fired up and ready to go.

The endorsement was released after the president met one-on-one in the Oval Office with Bernie Sanders. While Sanders won't be moving into that office himself, he has vowed to help Hillary Clinton defeat Donald Trump, and aides say Sanders will keep pushing for his progressive agenda.

Trump today met with prominent Republicans, but he's still having trouble with key figures in his own party, furious over his attacks on a Latino judge. Former candidates Ohio Governor John Kasich and Wisconsin Governor Scott Walker suggest they may not endorse Trump. Kasich says Trump has, quote, "gotten worse" since the race ended and House Speaker Paul Ryan voices dismay at what he calls Trump's antics, saying he hope Trump can fix this.

I'll speak with Democratic Senator Chris Murphy. And our correspondents, analysts and guests, they'll have full coverage of the day's top stories.

But let's begin with the breaking news. President Obama's endorsement of Hillary Clinton.

Let's go first to our White House correspondent, Michelle Kosinski. Michelle, this follow a meeting between the president and Bernie Sanders.

MICHELLE KOSINSKI, CNN WHITE HOUSE CORRESPONDENT: Yes. He didn't waste any time. To see how this played out was truly remarkable.

Here we have a presidential endorsement in a video, in a tweet that was put out by Hillary Clinton's campaign. We know that it was recorded on Tuesday, and it's clear that the White House wanted to wait for this long discussion today with Bernie Sanders.

Now, what we didn't hear from him was, "I'm bowing out. I'm now supporting Hillary Clinton." He approached those things, in essence, saying that now he will work with her and the White House toward their unified goal of defeating Donald Trump.

(BEGIN VIDEOTAPE)

BARACK OBAMA, PRESIDENT OF THE UNITED STATES: I congratulate Hillary Clinton.

KOSINSKI (voice-over): A presidential endorsement in the form of a slick campaign video released on Twitter.

OBAMA: I know how hard this job can be. That's why I know Hillary will be so good at it. In fact, I don't think there's ever been someone so qualified to hold this office.

KOSINSKI: Unity among Democrats is what they're all looking for, and that's the picture President Obama was careful to project, side by side with Senator Bernie Sanders today at the White House. But after the more than hour-long private discussion with President Obama in the Oval office, Sanders said he's not leaving the race just yet.

SEN. BERNIE SANDERS (D-VT), PRESIDENTIAL CANDIDATE: I will, of course, be competing in the D.C. primary, which will be held next Tuesday. We will continue doing everything that we can to oppose the drift which currently exists towards an oligarchic form of society.

KOSINSKI: He did suggest he sees the writing on the wall...

SANDERS: I will work as hard as I can to make sure that Donald Trump does not become president of the United States.

KOSINSKI: ... and plans to meet with Hillary Clinton.

SANDERS: I look forward to meeting with her in the near future to see how we can work together to defeat Donald Trump and to create a government which represents all of us and not just the 1 percent.

[17:05:07] KOSINSKI: The president going out of his way to praise the senator's hard-fought battle.

OBAMA: Just like eight years ago, there are millions of Americans, not just Democrats, who cast their ballots for the very first time. And a lot of that is thanks to Senator Bernie Sanders.

KOSINSKI: They want his supporters, many of whom right now are still saying, "Bernie or bust."

JOSH EARNEST, WHITE HOUSE PRESS SECRETARY: And you heard Senator Sanders say to all of you just a couple of hours ago how critically important it is that President Obama be succeeded by somebody who shares our values.

KOSINSKI: President Obama now free to hit the campaign trail and will in a matter of days.

OBAMA: I am fired up, and I cannot wait to get out there and campaign for Hillary.

KOSINSKI: Also on Twitter today, Donald Trump: "Obama just endorsed Crooked Hillary. He wants four more years of Obama, but nobody else does."

Hillary Clinton responding simply, "Delete your account."

(END VIDEOTAPE)

KOSINSKI: Sanders met with top Democrats in Washington today: the vice president, senators Chuck Schumer and Harry Reid. They want to hear him out. They want to find out what he wants. What does he see as best to rally as many supporters and attain that party unity? What we can expect to see in coming days from the White House is a kind of rollout of endorsements from the vice present and even from the first lady, Wolf.

BLITZER: We'll be watching closely with you. Thank you.

Let's bring in our senior Washington correspondent, Jeff Zeleny.

Jeff, Bernie Sanders made the rounds in Washington today, says he'll still compete in next week's primary. That's next Tuesday in Washington, D.C. What are his plans going forward?

JEFF ZELENY, CNN SENIOR WASHINGTON CORRESPONDENT: Wolf, he's still campaigning to a point. Yes, he'll be holding a rally tonight here in Washington, D.C., but he clearly has made a central pivot in this campaign.

Top of the order here is -- the first order of business is bringing his supporters on board, like Michelle just said there. He can join this. But the reality is, one of his attractive features to all these supporters was the fact that he was an outsider. So they know part of these discussions today were how can you bring these people on board so they don't either, A, stay home or, B, vote for an independent candidate or a Green Party candidate or someone else who is running?

ZELENY: So, Wolf, central to that is firing these people up. But I was struck by something that Bernie Sanders said right when he walked out of the White House about President Obama.

(BEGIN VIDEO CLIP)

SANDERS: Let me begin by thanking President Obama and thanking Vice President Biden for the degree of impartiality they established during the course of this entire process. What they said in the beginning is that they would not put their thumb on the scales and, in fact, they kept their word and I appreciate that very, very much.

(END VIDEO CLIP)

ZELENY: Wolf, that was a message there to his supporters, as well, because many of his supporters I talked to and covered at these rallies over the last many months believe that the White House was essentially supporting his opponent.

But he clearly made the point clear that he appreciated the respect that was shown to him. Wolf, in all respect, in winning 22 states, it's given him a bit of leverage, as well, here.

One of his key reasons for falling in line, he probably gets the team player of the day award here in Washington. But if Democrats should win control of the Senate, and that campaign will be happening simultaneously with the presidential campaign, Bernie Sanders will become a chairman of a major committee, likely the budget committee. Or at least potentially the budget committee. Those are some of the things that have been talked about.

That's one reason for him to go back to his day job with a smile on his face. But the Clinton campaign wants him to hit the campaign trail pretty quickly, and their meeting will likely come after the D.C. primary -- Wolf.

Yes. Let's see what happen after that. Thank you very much, Jeff Zeleny. Joining us now, Democratic Senator Chris Murphy of Connecticut. He

endorsed Hillary Clinton a year ago.

Senator, thanks for joining us. You believe it's time for Senator Sanders to step down and end his bid for the White House?

SEN. CHRIS MURPHY (D), CONNECTICUT: Well, I think you saw a clear pivot today. I think he's made it clear that his priority is defeating Donald Trump. There's one more primary to go, but I think the rhetorical shift has already happened.

And I think this is going to be a very, very powerful image to have, Hillary Clinton and Barack Obama campaigning together next week in Wisconsin and then perhaps the week after to have Hillary Clinton and Bernie Sanders campaigning together throughout the country.

To the extent that there are any lingering, undecided progressive or Democratic voters, they're going to come back to this party very, very quickly.

And Donald Trump is also going to be a tremendous unifying factor. His sort of proud overt racism is something that is going to drive all of Bernie's supporters, who may be a little reluctant to get into the tent, into Hillary's camp.

BLITZER: Are you saying within two weeks or so you think that Bernie Sanders and Hillary Clinton could be out on the campaign trail together? He campaigning for her?

MURPHY: I hope so. I think there's still going to be some important things that he wants to bring to the convention. But once we are finished with all of the voting, he and his supporters have said from the beginning that that would be the moment to come together.

[17:10:17] So I'm hopeful that Senator Sanders will be out there with Senator Clinton as quickly as possible. At this point, I think You can hear that there may not be a need to wait for the convention to have the unifying moment.

BLITZER: As you know, there's a lot of anger among many of Bernie supporters. What does Clinton -- what does President Obama, what does Hillary Clinton, what do they need to do to get those millions of, especially, young Bernie Sanders supporters on board to back Hillary?

MURPHY: Well, you know, as one of the youngest members of the Senate, which is, of course, a relative term, I care deeply about making sure that young voters turn out. And I think Senator Sanders' rhetoric has been so powerful to young voters, and maybe they haven't actually listened to some of the proposals that Hillary Clinton has made.

And maybe now, after he gets on board with her campaign, they'll hear the things she's saying about college affordability, and they'll hear some of the incredible progressive positions she has on issues like paid family sick leave.

Again, they are going to take a second look at Donald Trump. And younger voters, which care about tolerance, justice, inclusiveness are going to turn out for Hillary Clinton and turn out at the polls, also because they reject all of the divisive rhetoric that is going to still be the foundation for the Trump campaign.

BLITZER: Donald Trump has had a very successful Republican primary battle. He's run an unconventional campaign. Is there a fear Hillary Clinton could be underestimating Donald Trump? Is she ready to go up against his tactics?

MURPHY: I don't think she's underestimated Donald Trump. But I frankly don't believe folks who think this is going to be a runaway victory. This, unfortunately, is a country right now in which, you know, about 45 percent of the electorate is pretty solidly red, and about 45 percent is pretty solidly blue. And you're arguing over four years over about 10 percent in the middle.

So I don't think she's underestimating him at all. I think that's why you've seen her take the fight directly to him.

Now, this is going to be a challenge in that he is going to lie through his teeth. He is going to literally make up new things about her every single day. And so she's going to have to be very aggressive in that rapid response to correct the record. And she's, frankly, going to have to be helped by the media, which have done a much better job in the last several weeks of real time calling out Trump when he's flatly lying about himself or others.

BLITZER: Senator Murphy of Connecticut, thanks very much for joining us.

MURPHY: Thanks a lot.

BLITZER: Up next, Donald Trump is facing serious doubts and dissent within his own party. A few Republicans are upset at attacks on a Latino judge. They're demanding that he clean up his act. We'll have details.

(COMMERCIAL BREAK)

[17:17:32] BLITZER: We're following today's breaking news in the presidential race. President Obama announcing his endorsement of Hillary Clinton.

Gloria, today's endorsement was expected, but what's the significance of the timing, and what kind of impact do you think it will have?

GLORIA BORGER, CNN CHIEF POLITICAL ANALYST: Well, the timing is that it couldn't happen fast enough, as far as the president was concerned and as Hillary Clinton was concerned. I think they wanted to do it in the right way and orchestrate it properly. You see the president walking with Bernie Sanders very visibly at the White House, Sanders meeting with the vice president, talking to Hillary Clinton. You know, this was kind of soft diplomacy at work. And Bernie Sanders coming out and talking about defeating Donald Trump.

I think they want to get on the same page, and the president is just itching to get out there. I mean, we've seen that during his recent appearances. You know, he's been talking about Donald Trump, and you can see that he wants to get back out on the campaign trail. This is his legacy he's going to be talking about, and it all lies with Hillary Clinton winning this election.

BLITZER: Mark, what are you hearing about Senator Sanders and his future plans?

MARK PRESTON, CNN POLITICS EXECUTIVE EDITOR: Well, Wolf, certainly this meeting today with Barack Obama is being viewed internally as a very good meeting. They were happy with the outcome. They also were not surprised by this endorsement today. They pretty much knew it was coming.

But as far as going forward, you know, Bernie Sanders today is his day in many ways, in that people are looking at him as a real big team player at a time when they thought he would take this fight to the convention.

So now the question is, going forward the next couple of weeks, Wolf, is with Bernie Sanders giving to the Clintons his endorsement, eventually, and certainly not fighting it, will the Clinton campaign give back to Bernie Sanders. Couple of things that he'll be looking for is certainly policy issues. We know that the drafting platform committee right now is meeting here in Washington. There will be a few more meetings. See if he can push the Democrat agenda more to the left, as he would choose to do so.

And here's a little bit of insider information that is slightly interesting and we should be watching. There's been a lot made about whether Bernie Sanders can take out Debbie Wasserman Schultz as the chairperson of the Democratic National Committee. Maybe that will happen; maybe it won't.

But one thing we should be really looking for is to see whether the Clinton campaign will replace former Representative Barney Frank, as well as Connecticut Governor Daniel Malloy as the co-chairs of the rules committee. They are very upset -- the Sanders campaign is very upset at these two gentlemen, because they feel like they unjustly attacked Bernie Sanders to the Nth degree during this campaign, and they don't want these two gentlemen on the rules committee.

BLITZER: All right, Mark, interesting stuff.

Peter, President Obama will be joining Hillary Clinton out on the campaign next Wednesday in Wisconsin. How strongly would you expect him to come out against Donald Trump during that appearance and subsequent appearances?

PETER BEINART, CNN POLITICAL COMMENTATOR: Extremely strongly. And I think you'll see with a lot of humor as well, remember what Barack Obama did to Donald Trump at the White House Correspondents' Dinner a few years ago, where he kind of mercilessly mocked him, taunted him when Trump was in that birther phase of his.

I think one of the real advantages that Hillary Clinton has in this race is she has much stronger surrogates than does Donald Trump. And Elizabeth Warren has proven to be an effective surrogate. She has Bill Clinton. And Barack Obama, I think, will be a very, very effective attacker of Donald Trump.

Donald Trump goes on himself a lot that he does. There are a lot of Republicans, most Republicans, even those who have endorsed him, who don't necessarily want to go out there and be attached to him at the hip. And that's one of Hillary's real advantages.

BLITZER: Ryan, now that Hillary Clinton has the endorsement from the president, when would you expect a vice presidential pick on her part?

RYAN LIZZA, CNN POLITICAL COMMENTATOR: Well, the traditional time to do that is right before the convention. Of course, the conventions this cycle are a little bit earlier. So sometime in early July, I would expect that we'll start to hear that.

I mean, only a few big moments in a race where everyone is paying attention. Vice presidential pick, your convention and the debates. So it's one of the biggest decisions that Hillary Clinton has to make this year, and it will tell us a lot about what kind of campaign she's going to run.

Is she going to go left and play up the base? Or is she going to try and grab some of those disaffected Republicans and maybe point to someone who's more of a centrist?

BLITZER: It's an important decision she's got to make.

And very quickly, Ryan, I understand you have a new article coming out in "The New Yorker" magazine, in which a Republican senator has a potential bombshell, suggesting he or she isn't necessarily ruling out voting for Hillary Clinton. Tell us about that.

LIZZA: Yes. Senator Susan Collins of Maine tells me that she is opened to voting for Hillary Clinton. Now, she says it is unlikely. But this is the first really high-ranking Republican who has actually said that they could vote for Hillary Clinton.

Even Lindsey Graham and Ben Sasse, I mean, people that are really, really anti-Trump, have said they would never, ever consider voting for Hillary Clinton. But Susan Collins, the Maine moderate, she tells me that she is leaving that option open.

BLITZER: Very interesting stuff. And I know our viewers will want to read more about that in "The New Yorker" magazine.

Thanks very much, guys. Stand by. Just ahead, more on Donald Trump's meeting with big-money Republican donors just like the ones he repeatedly used to scorn during the primary campaign. Can he get them to open up their checkbooks right now?

(COMMERCIAL BREAK)

[17:28:04] BLITZER: Our breaking news: President Obama has endorsed Hillary Clinton for president, saying he doesn't think there's ever been someone so qualified for the office.

Donald Trump scoffed at the move, tweeting and I'm quoting him now, "Obama just endorsed Crooked Hillary. He wants four more years of Obama, but nobody else does."

Trump met today in New York with prominent Republicans and fundraisers.

Let's turn to our political reporter, Sara Murray. Sara, Trump is having some trouble firming up support within his own party. What's the latest?

SARA MURRAY, CNN POLITICAL CORRESPONDENT: That's right, Wolf. Donald Trump has been taking swipes this afternoon at Hillary Clinton on Twitter over this endorsement from President Obama.

But the reality is it's the Republicans that have been having this difficult time rallying behind their nominee, and we're hearing from more and more Republican leaders that they aren't sure if they're ever going to be able to back Trump.

(BEGIN VIDEOTAPE)

MURRAY (voice-over): Some Republicans just aren't ready to commit to Donald Trump.

GOV. JOHN KASICH (R-OH), FORMER PRESIDENTIAL CANDIDATE: Why would I feel compelled to support somebody whose positions kind of fundamentally disagree with.

MURRAY: Today, Trump's facing continuing fallout from his attacks against a federal judge, who Trump claimed was biased because of his Mexican heritage.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: I have a judge who is a hater of Donald Trump. A hater. He's a hater. His name is Gonzalo Curiel.

MURRAY: Now Wisconsin Governor Scott Walker is joining the chorus, suggesting he may not support the presumptive nominee after all and saying he's still waiting for Trump to renounce his prior criticism of the judge.

GOV. SCOTT WALKER (R-WI), FORMER PRESIDENTIAL CANDIDATE: I'll be watching, as others are closely, to see what happens going forward.

It's just sad in America that we have such poor choices right now.

MURRAY: Even House Speaker Paul Ryan, who's been privately pushing party unity, kept up his criticism today. REP. PAUL RYAN (R-WI), SPEAKER OF THE HOUSE: There's no justifying

those comments, and I was very clear about that. And I think we all -- everyone, not just Republicans, should disavow comments like that. And I'm glad he walked it back.

This is a long campaign with a long way to go, and he has a ways to go to give us a campaign that we can all be proud of.

[17:30:01] MURRAY: All of this as Trump's backers in Washington are struggling to calm the storm.

NEWT GINGRICH (R), FORMER HOUSE SPEAKER: Trump is learning how to be a candidate. Nobody has ever done what Trump has done. Come from totally outside, never run for office.

REP. CHRIS COLLINS (R), NEW YORK: Last week was a distraction. I believe Mr. Trump realizes that as well.

MURRAY: But Trump's tirades aren't the only thing giving Republicans heartburn. After nearly a year of refusing to fund raise and bashing big donors --

DONALD TRUMP (R), PRESUMPTIVE PRESIDENTIAL NOMINEE: Because once you take that money, you can take control.

MURRAY: Today Trump and his allies, including Republican national Committee chair Reince Priebus and New Jersey Governor Chris Christie huddled with top fundraisers.

The meeting coming as Trump dismisses the idea that he needs to raise a billion dollars for the general election and as some Republicans grow anxious about whether Trump can get raise the cash he needs to take on Hillary Clinton in the fall.

(END VIDEOTAPE)

MURRAY: Now fundraising hasn't been the only snag in building out Donald Trump's machinery. He has just a patchwork of aides across these battleground states that are going to be pivotal in the fall. In states like Ohio, Iowa, and Colorado. Trump doesn't have so much as a state director. Of course he and his aides are sort of brushing that aside saying look, the RNC has an organization, the state parties have staff, and we're going to lean on them to help us get through November -- Wolf.

Congressman, thanks very much for joining us. As you know, the speaker of the House, Paul Ryan, he called Trump's antics, he said they were distracting in a radio interview and then he went on to say this. He said, "But I think and hope and believe he can fix this to the point where he can hopefully run a campaign that we can all be proud of." But as you know, we've already seen how he runs a campaign defeating

19 Republican challengers. Why would you expect him to change his strategy in a general election campaign?

REP. DARRELL ISSA (R), CALIFORNIA: Well, because he has to. And one thing I know about business from my background is that the necessity of what it takes to win often changes how people work. You saw this in Donald Trump's prepared teleprompter speech delivered very well, very thoughtful speech, that he'd gone through line by line. If he continues to do that, he very quickly changes from that amateur candidate -- professional businessman amateur candidate, to somebody who begins to show the ability to deliver to the world these meaningful speeches.

BLITZER: Republican leaders are currently attending Mitt Romney's annual summit in Park City, Utah. You told me that Romney, quote, "need to get over the fact that somebody he didn't pick won." The big picture right now, where does Romney fit in along with the others who feel they've been alienated within their own party and they say they don't think they could ever support Trump?

ISSA: Well, as long as someone says they are not able to support somebody, it's kind of hard for them to be invited into the consulting circle. But assuming that Mitt Romney wants to make a difference, as so many of us do, including the speaker, what they have to be willing to engage with Donald Trump, expect to have the kind of direct access necessary to be heard and then give thoughtful comments to the candidate and see if in fact he takes the kind of advice that a first- time candidate absolutely needs.

BLITZER: Romney hasn't softened his criticism of Trump at all. He's actively tried, as you know, to recruit a third-party challenger. He says he won't run but do you think he might try to continue to stop Trump either by going after fundraisers or dedicating the support elsewhere, maybe even to Hillary Clinton?

ISSA: Well, that would be a sad thing for him to join Barack Obama in endorsing a candidate who is under active FBI investigation. You know, this is the first time in history that a seated president has said that they not only endorse but will campaign for somebody. Well, that president, attorney general and director of the FBI is actively investing valid criminal charges, ones in which people have taken the Fifth and they've granted immunity to get more information from people who clearly did wrong.

So, on one hand, we have a criminal situation. On the other hand, we have a first-time candidate trying to come from absolutely outside the system and who has succeeded in becoming our nominee and now needs to succeed in unifying the party. We saw H. Ross Perot come from outside and get 19 percent but to come from inside and unify the party is definitely a tall order and one that Donald Trump has to keep doing what he just did in that prepared speech and not what he's done at times off the cuff.

BLITZER: The Maine senator, Republican Susan Collins is leaving open the possibility of actually supporting Hillary Clinton over Donald Trump. This you heard Ryan Lizza just reported with his "New York" magazine article.

[17:35:01] Should more Republicans like Senator Collins follow? Should they actually go out and say they can't support Trump, they will vote for Hillary Clinton? What does the party do? Does the party have a plan to combat that?

ISSA: Well, I view all of us as Americans and if any American, be it Republican or independent, looks at Hillary Clinton and the Clinton operation that has gained net $371 million there at the Clinton Foundation, pays the family large amounts of money, Chelsea alone, $600,000 a year, for money gained leveraging their time in office and her time as secretary of state, commingling those activities, I think they make a decision that even if she's never indicted, that this is not somebody you want to give the ability to leverage public power to private gain because clearly Hillary and Bill Clinton and Chelsea have been doing that now for years quite successfully.

BLITZER: It was the "New Yorker" magazine that Ryan Lizza is reporting that in.

Elizabeth Warren, the senator from Massachusetts, the Democrat, she's doing her best to tie other Republicans right now to Donald Trump. It looks like this is going to be part of the Democratic playbook in this election cycle. What are the Republicans need to do to counter this if they, for example, want to get re-elected?

ISSA: Well, first of all, I don't think you run from the principles of the party. You can in fact differentiate anything that somebody says at the top of the ticket from what, in your district, is the reality and based on your voting record.

You know, Wolf, I've been traveling around the world and voting in Congress on behalf of our nation for almost 16 years. When I go home, I'm not talking about Donald Trump or even Hillary Clinton. I'm talking about the work we do in Congress and the work yet to be done not the least of which is dealing with the Middle East that is falling apart based on the wrong-minded thinking of the last eight years.

BLITZER: That's it. That's all the time we have.

Congressman, thanks very much for joining us.

ISSA: Thank you, Wolf.

BLITZER: Darrell Issa of California.

Tomorrow I'll be sitting down with the 2012 Republican presidential nominee Mitt Romney to discuss, among other things, his opposition to Donald Trump, his fundraising efforts for the Republican Party. It's an interview you won't want to miss tomorrow. 5:00 p.m. Eastern right here in THE SITUATION ROOM.

And coming up, more on the breaking news. A source now telling our CNN -- telling CNN the Democratic senator Elizabeth Warren plans to endorse Hillary Clinton tonight. Could she be Clinton's running mate?

And later, an American who joined ISIS and then changed his mind ran away from the terror group. He goes before a U.S. judge.

(COMMERCIAL BREAK)

[17:42:28] BLITZER: We're following the breaking news. Not only is President Obama endorsing Hillary Clinton, a source close to Massachusetts senator Elizabeth Warren says she will endorse Hillary Clinton tonight as well.

Warren, who many Democrats see as a potential Clinton running mate, also will be delivering a high-profile speech in which she will continue to attack Donald Trump.

Let's bring in CNN's Sunlen Serfaty.

Sunlen, Elizabeth Warren already seems to be filling a running mate's traditional role of political attack dog, isn't she?

SUNLEN SERFATY, CNN NATIONAL CORRESPONDENT: That's right, Wolf. She absolutely is. She's quickly become one of the most aggressive criticizers of Donald Trump. And tonight she is set to deliver one of the most aggressive attacks yet on him and tonight here as she readies that attack on Donald Trump, she's readying here endorsement of Hillary Clinton. That role as the party's de facto attack dog just one role of many.

SERFATY: Ripping into Donald Trump, calling him, according to prepared remarks, quote, "a loud, nasty, thin-skinned fraud who has never risked anything for anyone and serves nobody but himself. And serving up a scathing indictment of the entire Republican Party, attempting to tie all Republicans to their presumptive nominee and his controversies, like those over Judge Curiel's ethnicity.

Warren will say, quote, "Trump isn't a different kind of candidate. He's a Mitch McConnell kind of candidate. Trump is also House Speaker Paul Ryan's kind of candidate. Donald Trump chose racism as his weapon but his aim is exactly the same as the rest of the Republicans."

Warren, only a first-term senator, has acquired an oversized importance to Democrats and is now seen as pivotal in their quest to win the White House.

TRUMP: She's a goofas. So -- have you ever seen her? I mean, this woman, she's a basket case.

[17:45:06] SERFATY: Then there's Elizabeth Warren, the peacemaker. This perhaps most pressing right now. Warren is seen as key in helping to unify the Democratic Party after their fractious primary fight.

WARREN: Now I'm proud to be a Democrat in this election. Proud of the debate that Bernie Sanders and Hillary Clinton have had.

SERFATY: Warren is known as a progressive superstar with a powerful following, especially among supporters of Bernie Sanders and seen as essential to help bridge the divide between Sanders and Clinton supporters and perhaps between the candidates themselves.

SANDERS: Elizabeth Warren, I think, has been a real champion in standing up for working families and taking on Wall Street.

SERFATY: Then there's the question of Elizabeth Warren, the running mate, as Clinton starts her search for a vice president.

CLINTON: I'm not going to get into vice presidential choices but I have the highest regard for Senator Warren.

SERFATY: Progressive Democrats are energized.

UNIDENTIFIED FEMALE: I think that two women, whoever they may be, that would be fabulous.

SERFATY: Intrigued with the idea that not only that Clinton could call Warren up but at the prospect of an all-female ticket.

SEN. HARRY REID (D), MINORITY LEADER: I don't see why you couldn't have two women whoever they might be. I think they'd be as good as two men.

(END VIDEOTAPE)

SERFATY: And Warren has been a little bit coy when asked about all these buzz around her name out of potential vice presidential pick by Hillary Clinton. She says that she is squarely focused on her job in the Senate right now and at this time she's not looking for another job -- Wolf.

BLITZER: All right, Sunlen, thank you very much.

Coming up, a young man who grew up only miles from the nation's capital and was captured after fighting with ISIS goes to court facing decades behind bars.

(COMMERCIAL BREAK)

[17:51:15] BLITZER: A Virginia man who joined then defected from ISIS appears in federal court just hours after being flown back to the United States to stand trial.

Our Brian Todd was in the court when the young man stood before a judge.

Brian, what can you tell us?

BRIAN TODD, CNN CORRESPONDENT: Wolf, we were just a few feet from Mohamed Mohamad Khweis as he was led into the court by U.S. Marshals. I sat very close to his parents. Khweis came in looking thin and scared, gave his family a penetrating glance, then he heard the charges against him. His family was happy to see him alive, seemingly realizing just how dangerous his defection from ISIS was.

(BEGIN VIDEOTAPE)

UNIDENTIFIED MALE: Where are you from?

MOHAMAD KHWEIS, AMERICAN CHARGED WITH SUPPORTING ISIS: United States.

TODD (voice-over): He is the first American fighter with ISIS to be captured on the battlefield. Tonight 26-year-old Mohamad Khweis is back in America charged with providing material support to ISIS. A terror group that U.S. officials say Khweis wanted to join and then soured off.

KHWEIS: I didn't really support their ideology and that's -- at that point that's when I decided I needed to escape.

TODD: After Khweis' dramatic capture by Kurdish forces in March, he did an interview with Kurdish TV. We don't know whether he was coerced at the time of filming. He talked about how he flew from the U.S. to London in December, made his way to Turkey. While there, Khweis said, he met up with at least two women, one of whom helped him get into Syria, then Iraq.

KHWEIS: I made a bad decision to go with the girl and go to Mosul. At the time I made a decision to go because I wasn't thinking straight and on the way there I regretted, I wanted to go back.

TODD: Prosecutors say an ISIS militant once asked Khweis if he wanted to be a suicide bomber. They say Khweis answered yes, but he thought the question was just to test his commitment to ISIS. Khweis' lawyer downplayed it.

JOHN ZWERLING, MOHAMED KHWEIS' ATTORNEY: I have no reason to think he wanted to end his life for any particular reason.

TODD: When CNN went to Khweis' family's home in March, his father was upset with the media attention. Today he again didn't feel like saying much.

JAMAL KHWEIS, MOHAMAD KHWEIS' FATHER: I'm happy to see him today.

TODD: Khweis graduated from Edison High School in Alexandria, Virginia, in 2007. Harrison Weinhold, a friend from high school, says Mohamad Khweis was a normal teenager who made fun of people who were religious.

(On camera): What do you make of this news about your friend?

HARRISON WEINHOLD, HIGH SCHOOL FRIEND OF MOHAMAD KHWEIS: It's really -- it's upsetting, really sucks, something you feel for his family. This is not something that you would ever think would happen. Definitely wasn't the type of person, wasn't an angry person, wasn't a -- you know, an outcast by any means. He was just a normal guy. We did normal stuff in high school.

TODD (voice-over): Tonight, analysts say Khweis could be a gold mine for U.S. intelligence.

PETER BERGEN, CNN NATIONAL SECURITY ANALYST: He certainly has interesting information about the recruiting process, training process, the other recruits that were there.

(END VIDEOTAPE)

TODD: Khweis' attorney would not say whether his client would cooperate with prosecutors or with U.S. intelligence. Now if he's convicted, he could get 20 years in prison and a $250,000 fine. But our analyst Peter Bergen says Khweis at this point is just lucky to be alive. Bergen says when ISIS catches fighters who try to defect, they kill them -- Wolf.

BLITZER: Brian, have you found anything else out why he became attracted to ISIS to begin with?

TODD: Wolf, there's very little to indicate specifically what attracted him. Prosecutors say he did admit watching several ISIS videos before he left the U.S., including videos of executions. They say he communicated with ISIS recruiters. They say he told the FBI that he, quote, "gave himself to ISIS," and that they controlled him. But Khweis' lawyer cautioned all of us today.

[17:55:03] He's saying that everything is not as it appears in the government's court records and he looks forward to telling their side of it.

BLITZER: Brian Todd, thank you very much for that report.

Coming up, after breaking the news to Bernie Sanders, President Obama delivers his big endorsement of Hillary Clinton, saying that he's fired up to join her out there on the campaign trail.

And Donald Trump, he's still facing trouble within his own party. Some key Republican leaders are withholding their support, warning Trump to basically clean up his act.

(COMMERCIAL BREAK)

BLITZER: Happening now. Breaking news. Seal of approval. President Obama endorses Hillary Clinton moments after meeting with Bernie Sanders at the White House. President Obama and Hillary Clinton now scheduled to hit the campaign trail together. Can they unify a divided Democratic Party?